Continuous metal matrix composite consolidation

ABSTRACT

A method for the fabrication of large metal matrix composite structures comprising the continuous joining by brazing or welding of aluminum matrix tape using an infrared laser to melt the surface of the tape while applying pressure to the tape and simultaneously contacting it with previously applied tape layers on a rotating mandrel. The apparatus utilized to accomplish this fabrication process may include a variety of pre and post-contact heaters and preferably includes instruments for the continuous monitoring and control of the process.

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/733,566 filed Dec. 8, 2000 now U.S. Pat. No. 6,455,804.

This invention was made with Government support under contract numberF33615-99-C-5203 awarded by the Air Force. The Government has certainrights in the invention.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for thecontinuous consolidation of metal matrix composite materials and moreparticularly to methods and apparatus for the consolidation of aluminummatrix, ceramic fiber reinforced metal matrix composites in prepeg tapeform.

BACKGROUND OF THE INVENTION

The advantageous properties of metal matrix composites, especiallyaluminum matrix composites that incorporate ceramic reinforcing fibersare well known and recognized in the art and include high specificstrength, high specific stiffness, maintenance of properties at extremesof high and low temperature and their resistance or lack of outgassingin a vacuum which is a major shortcoming of many competitive materials.These properties are of particular importance in aviation and spacevehicle and structural applications. In fact, it has been estimated thatthe use of aluminum matrix composites of this type in, for example,launch vehicles could reduce their weight by as much as 30%, thusincreasing their available payload by a like amount.

What is inhibiting the use of such materials in launch and similarvehicles, is a cost effective manufacturing method for the production oflarge structures from these materials. The provision of such a methodwould permit such applications for these materials and provide all ofthe accompanying attendant benefits to such use.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide a methodfor the manufacturing of large structural members from aluminum metalmatrix composites (AMCs).

It is another object of the present invention to provide a costeffective such manufacturing method.

It is yet another object of the present invention to provide apparatusfor the implementation of such a manufacturing method.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method for thefabrication of large AMC structures comprising the continuous welding orbrazing of an aluminum matrix bare or braze-clad fiber reinforced tapeusing an infrared laser to melt the aluminum or the braze cladding onthe tape while applying pressure to the tape and simultaneouslycontacting it with previously applied tape layers on a rotating mandrel.The apparatus utilized to accomplish this fabrication process mayinclude a variety of pre and post-contact heaters and preferablyincludes instruments for the continuous monitoring and control of theprocess.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective view of apparatus suitable for themanufacture of AMC structures in accordance with the process of thepresent invention.

FIG. 2 is a schematic depiction of the area of contact between themandrel surface, and the incoming prepeg tape at the point ofapplication of infrared laser radiation in accordance with the processof the present invention.

FIG. 3 is a schematic depiction of the area depicted in FIG. 2 exceptunder the preferred alternative condition where the prepeg tape is notbraze coated prior to consolidation.

DETAILED DESCRIPTION

The present invention provides a method for the cost effectivefabrication/manufacture of large structural members of aluminum metalmatrix composites. The feedstock for the process is a metal matrixcomposite (MMC), specifically an aluminum matrix composite (AMC), inprepeg tape form comprised of alumina (Al₂O₃) fibers in analuminum/aluminum alloy matrix. The prepeg tape can be bare or coatedwith a “brazing” alloy, i.e. an aluminum alloy having a lower meltingpoint than the aluminum matrix of the prepeg tape, prior to applicationin the process of the present invention. Fabrication is accomplished byapplying the bare or braze material coated prepeg tape to a rotatingmandrel with the application of pressure while simultaneously meltingthe braze coating or the surface of the bare metal, in the case of annon-braze coated tape, at the junction between the prepeg tape and themandrel surface using a laser, preferably an infrared or diode laserthat provides very limited and very localized heating and melting of thebraze coat or surface layer of aluminum. The laser beam of infraredradiation preferably has a rectangular cross section to enhance heatingefficiency in the area of the junction. As will be seen from thedetailed description that follows, a variety of pre and post-contactheaters and process control devices are preferably used to control andmonitor the process. The braze-coated feedstock just described can beprefabricated at a remote location and provided in coil form, or, asdescribed hereinafter, can be prepared just prior to fabrication bycoating the AMC prepeg with the braze coat in line just prior toexposure to the laser radiation and application to the mandrel.

While any number of techniques such as spraying (thermal, arc, plasma,etc.), surface alloying, etc. can be used to apply the lower meltingbraze coating to the prepeg tape, in the case where the braze coating isapplied in line with the consolidation operation, the prepeg tape ispreferably guided through a pot of molten brazing, i.e. lower melting,metal, extracted from the pot of metal through a coating thicknesscontrol device such as a die or air knife, and then through a coolingchamber to solidify the coating. Preferably, the pot of molten metal isequipped with an ultrasonic pulse inducing element comprising a powersupply, a transducer and a probe to facilitate coating of the matrix ofthe prepeg tape with the braze coating. When used, the ultrasonic probeis inserted into the pot of lower melting molten metal it produces acavitation field that results in pressure waves that reduce the contactangle and improve the wetting of the lower melting material to theprepeg. The cooling chamber can be highly sophisticated, but can be assimple a metal tube through which is flowed a chilled gas such asnitrogen and through which the braze coated prepeg travels on exit fromthe coating pot and the thickness control device.

Referring now to FIG. 1, the consolidation apparatus 10 of the presentinvention, comprises a rotating mandrel 12 supported on legs 14 (or anyother suitable support system), a laser 16 that directs a beam ofinfrared radiation 18 to the junction 20 between braze coated or bareprepeg tape 22 and surface 24, a carriage unit 26 that supports andimparts lateral traversing motion to compaction wheel 28, pre-heaters 30and post heater(s) 32. According to a preferred embodiment of thepresent invention, an optical pyrometer 33 can be used to monitor thetemperature at junction 20 and the signal therefrom used to controleither the mandrel rotation an/or carriage unit traverse speeds or theintensity of laser 16, to thereby control the temperature of the moltenbraze coating 36 (see FIG. 2) or aluminum (see FIG. 3) that occurs atjunction 20.

Referring now to FIG. 2 that schematically depicts a side view ofconsolidation apparatus 10 and shows the relative positions of laser 16,infrared radiation beam 18, compaction wheel 28, mandrel 12 and incomingbraze-coated prepeg tape 22 at junction 20, it is readily observed thatat junction 20, there exists a “front” of molten metal 34 that comprisesthe molten or liquid form of braze coating 36 on prepeg tape 22. Front34 is produced by the localized heating induced by the impact ofinfrared raditation beam 18 upon the surface of braze coating 36. Itmust be noted, that although not specifically depicted in FIG. 2,surface 24 of mandrel 12 includes at least one wrap of previouslyapplied prepeg tape 22 to which incoming feedstock prepeg tape 22 isadhered as braze coating 36 melts due to the localized and controlledheating action of infrared radiation beam 18, and subsequently cools asit is removed from the area of front 34 due to rotation of mandrel 12 inthe direction shown by arrow 38 thereby building serial overlying layersof AMC joined to each other by alternating layers of braze material 36.Simultaneously with the creation of front 34 and the movement of prepegtape 22 in the direction indicated by arrow 38, compaction wheel 28pushes prepeg tape 22, and consequently associated melted braze coating36, into intimate contact with surface 24 on mandrel 12 causing prepegtape 22 to adhere firmly thereto. The specific conditions under whichsuch fabrication can occur are described in greater detail hereinafter.

Consolidation apparatus 10 fundamentally comprises a 2-axis filamentwinder of the type used in the fabrication of polymer matrix composites.According to a preferred embodiment, mandrel 12 can be up to 48 incheslong and up to about 36 inches in diameter. Of course, largerdimensioned devices can be used in those cases where larger structuralmembers are being fabricated. The rotational movement of mandrel 12 andthe linear traverse of compaction wheel 28 on carriage unit 26 arecontrolled and coordinated by means of “Pattern Master” software or thelike that are supplied with the filament winder unit, or custom deignedand implemented if a specific non-standard wrap pattern is required ordesired.

Laser 16 preferably comprises a stacked multi-bar infrared laser. Anarray of optical lenses 38 are used to shape infrared radiation beam 18into a rectangular pattern that matches the cross-sectional dimension ofprepeg tape 22. According to a preferred embodiment of the invention,laser 16 is powered by a DC power supply capable of delivering 75 ampsto the preferred stacked multi-bar diode laser 16. Laser 16 in thisconfiguration is designed to operate in a continuous wave mode at apower of up to 500 watts. In the embodiment depicted in FIG. 3 wherein aseparate braze coating layer is not applied but rather a slightlythicker (by perhaps one or two thousandths of an inch) of metal thatforms the matrix is used, a higher power laser is necessary due to thegenerally higher melting point of the matrix material as compared to theapplied braze coating described herein. Thus, for alternative suchprocesses it is preferred that the laser exhibit a power of betweenabout 100 and about 1500 watts and preferably between about 500 andabout 1000 watts. Water cooling of the laser head is required tomaintain the life of the diodes and is conventionally accomplished bymeans of a water-to-air chiller unit (not shown). Multi-bar diode lasersof this type are commercially available from Opto Power Corporation,3321 E. Global Loop, Tucson, Ariz. 85706.

Mandrel 12 must, of course be collapsible or otherwise removable oncethe finished structure is completed by completion of the wrappingoperation. Similarly, surface 24 of mandrel 12 should be of a materialthat will resist adhesion to melted and cooled braze coating 36, ormatrix metal when performed as described in connection with FIG. 3, andsimultaneously minimize conductive heat loss from the parts duringfabrication to provide better and more accurate process control,although in the latter case, alternative process controls may be used tominimize the effects of the material on surface 24 on thebrazing/welding process. In one embodiment of the present invention, asuitable ceramic tube fabricated from shale and fire clay was cut intothree segments and attached to a chuck arrangement to allow forexpansion and contraction. In this case, the amount of material removedduring the cutting operation was minimized to prevent surface 24 frombeing out of round.

Referring now to FIG. 3, it is to be noted that the only fundamentaldifference between the embodiment depicted therein and in FIG. 2 is thatin the embodiment depicted in FIG. 3 no braze coating 36 andconsequently brazing material front 34 is present, rather a matrixmaterial front 34A that is in effect a weld bead of molten matrixmaterial derived from both opposing surfaces of prepeg tape 22 is formedto consolidate the sequentially applied layers of prepeg tape 22 asmandrel 12 is rotated. Of course, in such an instance, no mechanism isrequired for the application of braze coating 36. In all other respects,except those specifically differentiated hereinabove, the method andapparatus of this embodiment are identical to that of the previouslydescribed embodiment wherein a brazing layer is utilized. In theembodiment depicted in FIG. 3, it is fair to characterize the layerbonding process as one of laser welding of the sequentially appliedlayers of prepeg tape 22.

As shown in FIG. 1, immediately after junction 20 prepeg tape 22 iscontacted on its reverse side 40 by compacting wheel 28 to accomplishconsolidation. As with surface 24 of mandrel 12, compacting wheel ispreferably fabricated from a ceramic material to minimize conductiveheat loss from junction 20 during consolidation. A highly preferredmaterial for compaction wheel 28 is zirconium phosphate which exhibitsthese and other suitable properties. Of course, suitable alternativeprocess controls can make the selection of materials for compactionwheel 28 less critical. Compaction wheel 28 is arranged to ride at topdead center of mandrel 12 and is guided in its movement by carriageassembly 26. Compaction wheel 28 in addition to providing compressiveenergy for consolidation also has a second important function, in thatit provides a V-shaped cavity at junction 20 thereby reducing reflectivelosses by trapping some of the infrared radiation of beam 18 andcreating a “multiple bounce” situation where most of the incomingradiation is used for heating and less of such radiation is lost due toreflection from the various surfaces at junction 20.

Preheat lamps 30, and where used post heat lamp(s) 32 preferablycomprise reflector lamps as line sources of infrared energy to preheator post heat prepeg tape 22 prior to or after exit from junction 20.Preheat lamps 30 preferably heat prepeg tape 22 to a temperature ofabout 500° F. in order to reduce the heating load on laser 16. As willbe obvious to the skilled artisan, such preheating may not be requiredif a higher powered laser is used. Post heating lamp(s) 32 are similarlyconfigured, and if and where applied can be used to control the cooldown of prepeg tape 22 as it exits junction 20 to reduce the thermalstresses that may be induced by the brazing/welding process.

According to another alternative preferred embodiment of the presentinvention, a rotary ball vibrator 42 that induces vibration in the rangeof from about 1000 to about 25000 vibrations per minute is added toconsolidation apparatus 10 to provide a more thorough mixing of moltenbraze alloy front 34 at junction 20. Rotary ball vibrator 42 is attachedto a metal rod 44 that contacts prepeg tape 22 just before it entersjunction 20. The presence of rotary ball vibrator 42 causes prepeg tape22 to vibrate at the same frequency as vibrator 42 which in turn inducesoscillations in front 34 at junction 20. Thus, these oscillations occurin junction 20 as prepeg tape 22 is addressed by compaction wheel 28.

According to yet another alternative preferred embodiment of the presentinvention, a flow of inert gas is applied over the heated area atjunction 20 to minimize the formation of oxides in front 34 duringbrazing/welding. Free flowing argon, nitrogen or the like inert gasdirected to the area of junction 20 appears to provide such benefit.

Optical pyrometer 33 may be included to provide temperature feedbackinformation to the control circuits of laser 16 thereby assuring thatthe appropriate amount of heat is being applied at junction 20 toachieve satisfactory melting of braze coating 36, or welding in thenon-braze coated embodiment, and consolidation as described above.

Finally, at least in process development and refinement situations, itcan be desirable to include a video camera (not shown) to closelymonitor the area of junction 20 to obtain the appropriate operatingparameters for a specific given prepeg tape 22 and braze coating 34composition.

In practice, the method of the present invention is carried out usingthe above-described apparatus 10 by first wrapping an initial turn of asuitable prepeg tape of, for example, pure aluminum, 1100 alloy aluminumor any other suitable aluminum, titanium, magnesium etc. metallic matrixcontaining a ceramic reinforcing material, for example, Nextel 610™aluminum oxide 1500 denier fibers commercially available from the 3MCorporation, Minneapolis, Minn. According to a specifically preferredembodiment of the present invention the prepeg tape, whatever itscomposition, is about 0.5 inches wide, 0.015 inches thick with arectangular cross section about mandrel 12. As will be apparent to theskilled artisan, the particular dimensions of prepeg tape 22 are largelya matter of the particular consolidating apparatus 10 that is used, i.e.its dimensions, and, within easily understood limitations, the processis feasible with virtually any width and thickness of prepeg tape 22with appropriate alternative selection of consolidation conditions.Prepeg tape 22 is provided as a coil on a payoff for continuous feeding.Consolidation apparatus 10 is then activated. Mandrel 12 begins to turn,laser 16 is focused on junction 20 and prepeg 22 is fed into junction 20for consolidation by compacting wheel 28. The specific processconditions are largely a matter of choice as dictated by the materialsbeing consolidated (the AMC matrix alloy and the braze coatingcomposition), the power of laser 16, the rotational speed of mandrel 12etc. However, in the case of fabrication of the above-described prepegtape bearing braze coatings of the types referred to in the examplesbelow, melting temperatures in the range of from about 375 to about1200° F. produced by a suitable laser operating at between about 100 andabout 500 watts and prepeg tape feed rates on the order of between about0.65 and 1.50 inches/sec. have been found useful and appropriate. In thecase where prepeg tape 22 bears no braze coating 36 the power of thelaser may be required to be significantly higher, for example, betweenabout 500 and 1500 watts.

EXAMPLES

The following examples when considered in conjunction with the foregoingdetailed description will serve to better illustrate the successfulpractice of the present invention.

Examples 1–4

Prepeg tapes comprising Nextel 610™ fibers in pure aluminum wereconsolidated as described hereinabove using the following braze coatingsand under the following tabularly presented operating conditions:

Braze Braze Laser Tape Feed Coating Temperature Power Rate 1) 96.5Sn/3.5 Ag 430–500° F. 426 Watts 0.70 inches/sec. 2) 70 Sn/30 Zn 389–707°F. 110 Watts 1.06 inches/sec. 3) 84 Zn/11 Al/5 Cu 715–845° F. 268 Watts0.87 inches/sec. 4) 88 Al/12 Si 1070–1220° F. 373 Watts 1.27 inches/sec.

Under each of the foregoing conditions, satisfactory consolidated roundstructural shapes of the prepeg material indicated were fabricated.

As the invention has been described, it will be apparent to the skilledartisan that the same may be varied in many ways without departing fromthe spirit and scope of the invention. Any and all such modificationsare intended to be included within the scope of the appended claims.

1. A method for the fabrication of structural members of metal matrixcomposites comprising: providing a rotating mandrel having a mandrelsurface addressed by a linearly traversing compaction wheel; feeding ametal matrix prepeg tape having opposing surfaces between said mandrelsurface and said compaction wheel, wherein as the mandrel is rotatedsaid metal matrix prepeg tape is successively layered completely aroundsaid mandrel, thereby wrapping the prepeg tape around the mandrel, andwherein said metal matrix prepreg tape fed between said mandrel and saidcompaction wheel defines a V-shaped junction; impacting said metalmatrix prepeg tape with a beam of infrared radiation at said junction tomelt at least one of said opposing surfaces in said junction;simultaneously with the impacting of said beam of infrared radiation insaid junction, rotating said mandrel so as to take up said prepeg tapeas said at least one melted surface cools and solidifies and pressingsaid compaction wheel against said prepeg tape so as to causeconsolidation of said prepeg tape with previously applied layers of saidprepeg tape on said mandrel surface, thereby forming a consolidatedmetal matrix composite around the mandrel; and removing said mandrelfrom said consolidated metal matrix composite to provide a metal matrixcomposite structural member.
 2. The method of claim 1 wherein saidprepeg tape comprises a matrix of aluminum or an alloy of aluminumencompassing ceramic fibers.
 3. The method of claim 2 wherein saidceramic fibers comprise aluminum oxide.
 4. The method of claim 1 whereinsaid infrared radiation is produced by a stacked multi-bar infraredlaser.
 5. The method of claim 4 wherein said stacked multi-bar infraredlase includes optical lenses that shape the infrared radiation into apattern that matches the cross sectional dimensions of said prepeg tape.6. The method of claim 1 wherein said mandrel is collapsible so as topermit removal of multiple layers of applied prepeg tape therefrom whenfabrication is complete.
 7. The method of claim 1 wherein said mandrelsurface and said compaction wheel both comprise the same or differentceramic materials.
 8. The method of claim 1 wherein said prepeg tape ispreheated with infrared reflector lamps prior to entering said junction.9. The method of claim 1 including the further step of providing anoptical pyrometer that views said at least one melted surface in saidjunction and provides temperature feedback information for controllingthe power of said infrared radiation or the speed of rotation of saidmandrel.
 10. The method of claim 1 further including the application ofvibratory energy to said prepeg tape prior to entry into said junctionat a frequency of between about 1000 and 25000 vibration per minute. 11.The method of claim 1 wherein said prepeg tape comprises a matrix of1100 aluminum alloy or pure aluminum having aluminum oxide fibersembedded therein, said infrared radiation is provided by an infraredlaser operating at a power level of from about 100 to about 1500 wattsand said prepeg tape is fed at a rate of between about 0.65 and about1.5 inches/sec.
 12. The method of claim 11 wherein said prepeg tape isabout 0.5 inches wide and about 0.15 inches thick.
 13. The method ofclaim 1 wherein at least one of said opposing surface comprises a brazecoating.